Leeds U.S. Pat. Nos. 2,777,824 and 3,055,297 disclose marking structures made of highly porous plastic material, the pores of which are of microscopic proportions and are filled with a marking fluid such as an ink. Structures made in accordance with the teachings of the Leeds patents have experienced a high degree of commercial success as hand stamps, stamp pads, and also as ink rolls such as are used for applying ink to printing members in automatic printing equipment. Such structures are advantageous because of their long life, both in length of time and in numbers of operations or impressions, and because they operate well without the necessity of repetitive re-inking of the marking surface. Such structures apply ink in a uniform and reliable fashion. For example, hand stamps including plates made using such structures, which have various characters or designs molded at their surface, provide sharp and uniform impressions with high definition and uniformity. As stamp pads, such structures place a uniform layer of ink on a marking device, such as a rubber stamp, and are reliable over long periods of time. Similarly, as ink rolls, such structures provide uniform inking of printing members and exhibit fast recovery to facilitate repetitive use over long periods of time.
One problem with the method of making the marking structures disclosed in Leeds U.S. Pat. Nos. 2,777,824 and 3,055,297, is the time required to produce these structures. For example, in the case of hand stamps, the total processing time may be a period on the order of about 15 hours. A lengthy period of time is primarily required for a "curing" step, in which the molded structure reaches an initial dimensional stability and excess ink flow from the structure is stabilized. The present invention reduces the processing time to about two hours for a typical hand stamp, primarily because no lengthy "curing" step is necessary.
An additional concern with the marking structures made using the disclosures of Leeds is that the process is complex and requires skilled workers to produce acceptable marking devices. This leads manufacturers of these devices to centralize their manufacture. This is a problem for hand stamps which are custom made to order. Because of the time required, the prior process could not prepare stamps on a "while you wait" basis but required a one to two day wait.
Known methods of making marking structures generally involve an initial step of making a mold having indentations or cavities corresponding to a relief pattern to appear on the finished marking structure. The mold is made of a material which can partially absorb marking fluid from the finished marking structure. Since it is necessary to use an excess amount of marking fluid in order to make the marking structure, it is necessary to make the mold from an absorbent material to remove the excess ink from the marking structure as it cures in the mold.
A premix containing a thermoplastic resin and a marking fluid, such as an ink, is then placed in the mold to fill the indentations and form a layer of premix therein. The resulting premix-filled mold is then placed in a press, and subjected to heat and pressure sufficient to form the ink-impregnated microporous structure which is suitable for making a self-inking (self-replenishing) hand stamp. The open-celled, skinless nature of the microporous structure allows a small but generally constant flow of ink from the marking structure. Thus, as ink is removed from the surface of the relief pattern on the marking structure when an impression is made, ink from the interior of the marking structure flows to the surface in sufficient quantity to allow formation of further images having substantially the same brightness and clarity as the initial image.
In the foregoing known method, it is important to avoid using an non-absorbent mold or matrix board. Under the heat and pressure used in the step of making the microporous structure, a non-absorbent mold would ruin the marking structure. The present invention provides a method for making microporous marking structures which eliminates some of the disadvantages of this known method.